The present invention generally relates to heat shields, and more particularly, to a lightweight insulated heat shield for minimizing heat transfer to, for example, a spacecraft structure during atmospheric reentry.
Many different heat shield concepts for minimizing heat transfer to a spacecraft structure during atmospheric reentry are known. Traditional heat shield concepts, such as heat shields comprised of carbon phenolic are relatively heavy and therefore introduce a significant weight penalty to the spacecraft. Conventional carbon-carbon heat shields have a relatively high thermal conductivity which may cause overheating at the heat shield-spacecraft structure interface during reentry. Some available heat shield materials that can withstand moderate reentry heating environments, such as SLA-561V and SIRCA, are unable to survive more severe reentry environments involving high surface pressures (e.g., greater than 0.2 atm) and high heating (e.g., greater than 200 W/cm2). Other materials currently being used for high temperature applications, such as phenolic impregnated carbon ablator (PICA), have surface pressure and heat shield manufacturing limitations.
In view of the foregoing, one objective of the present invention is to provide a lightweight heat shield for thermal protection applications, such as, for example, minimizing heat transfer to a spacecraft structure during atmospheric reentry.
Another objective of the present invention is to provide a lightweight heat shield capable of withstanding severe reentry environments.
These and other objectives and advantages are achieved by the inventive materials concept for atmospheric reentry heat shields disclosed by the present invention. According to one aspect of the present invention, a heat shield includes an outer heat resistant layer comprised of an ablative first material. The outer heat resistant layer is backed by an inner insulating layer comprised of an insulating second material. The outer and inner layers are bonded to one another by a middle layer formed by disposing at least one layer of a phenolic loaded third material between the outer and inner layers and heating all three layers simultaneously to remove phenolic volatiles from the third material.
In one embodiment, the outer layer comprises carbon-carbon laminate and the inner layer comprises carbon foam. The outer carbon-carbon laminate layer may include an oxidation resistant surface treatment or coating on an outer surface thereof. The middle layer bonding the outer and inner layers together is formed by disposing at least one layer of phenolic loaded carbon scrim cloth between the outer and inner layers and heating all three layers simultaneously to remove phenolic volatiles from the carbon scrim cloth. This results in a middle layer comprised of carbon scrim cloth and phenolic char (i.e. the components of the phenolic resin remaining after removal of the phenolic volatiles). The carbon scrim cloth/phenolic char middle layer provides a compliant bond between the outer and inner layers that can withstand movement without substantial separation between the outer and inner layers as the heat shield undergoes thermal expansion and other stresses during reentry. The bond provided by the middle layer is compliant because all three layers are thermally compatible. The three layers are thermally compatible as a result of having similar coefficients of thermal expansion due to their carbonaceous nature.
In order to further enhance the strength of the bond provided by the middle layer between the carbon-carbon outer layer and the carbon foam inner layer, a layer of carbon scrim cloth may be co-processed with the carbon-carbon laminate when preparing the outer carbon-carbon shell. The carbon scrim cloth may be included as the innermost surface in the carbon-carbon laminate outer layer. This further enhances the bond formed by the middle layer because the carbon scrim cloth provides the outer layer with a rougher inner surface than would otherwise be provided by an outer layer comprised of only carbon-carbon laminate.
In addition to carbon-carbon, the ablative first material comprising the outer layer may, for example, be a carbon-phenolic or a ceramic matrix composite material. Regardless of its composition, the outer layer should be sufficiently thick to provide complete coverage of the inner insulating layer during atmospheric reentry. The appropriate thickness will vary depending upon the expected amount of surface recession of the outer heat resistant layer for a given set of mission parameters (e.g. anticipated surface pressures and heat flux) plus an additional margin necessary to ensure that a sufficiently thick outer layer remains throughout reentry in order to protect the inner layer from gas flow through the outer layer. For example, a carbon-carbon outer heat resistant layer may be between 0.10 and 0.25 inches thick depending upon mission parameters. As another example, a ceramic matrix composite outer layer may be as thin as 0.05 inches, depending upon mission parameters. Also, the outer layer is preferably sufficiently dense in order to inhibit the permeation of gases therethrough during reentry. For example, a carbon-carbon outer heat resistant layer preferably has a density of at least 1.6 grams per cubic centimeter.
In addition to carbon foam, the insulating second material comprising the inner layer may, for example, be reticulated vitreous carbon, graphite felt, ceramic foam, ceramic felt, impregnated microspheres of carbon, or impregnated microspheres of ceramic. The inner insulating layer reduces the rate of heat transport to the spacecraft structure. In this regard, the inner insulating layer will typically be substantially thicker than the heat resistant outer layer in order to ensure that the temperature at the heat shield spacecraft interface is maintained at or below a desired temperature during reentry. Thus, the insulating second material comprising the inner layer preferably has a lower density than the ablative first material comprising the outer layer so that the heat shield remains lightweight.
In addition to carbon scrim cloth, the phenolic loaded third material may, for example, be phenolic loaded carbon felt, ceramic scrim cloth or ceramic felt. Whether carbon scrim cloth or felt or ceramic scrim cloth or felt is appropriate depends upon the nature of the first and second materials comprising the outer and inner layers, respectively. In this regard, when the first and second materials are carbonaceous in nature, the third material is preferably carbon scrim cloth or felt, whereas, when the first and second materials are ceramic in nature, the third material is preferably ceramic scrim cloth or felt. Carbon or ceramic felt, as appropriate for the first and second materials, may provide an even more compliant bond between the outer and inner layers of the heat shield than carbon or ceramic scrim cloth.
In one embodiment, the insulating inner layer may be comprised of a plurality of blocks of the insulating second material. The blocks of the insulating second material are bonded to the outer layer by the middle layer, and adjacent blocks of the insulating second material may also be bonded to one another. For example, adjacent blocks may be bonded to one another by disposing one or more layers of the phenolic loaded third material therebetween, with the phenolic volatiles being removed when the heat shield is heated to remove the phenolic volatiles from the middle layer. As another example, in lower temperature applications, adjacent blocks may be bonded to one another using a low temperature adhesive in combination with one or more layers of scrim cloth between the blocks. The blocks of the insulating second material may be machined to match a spacecraft structure to which the heat shield is attachable.
According to a further aspect of the present invention, a method for use in constructing a heat shield includes the steps of preparing an outer heat resistant layer comprised of an ablative first material and attaching the outer heat resistant layer to an inner insulating layer comprised of an insulating second material. In this regard, the outer heat resistant layer is attached to the inner insulating layer by disposing at least one layer of a phenolic loaded scrim cloth or felt material between the outer heat resistant layer and the inner insulating layer and heating the outer heat resistant layer, the layer(s) of the phenolic loaded scrim cloth or felt material, and the inner insulating layer simultaneously to remove phenolic volatiles.
When the ablative first material comprises carbon-carbon, the step of preparing the outer heat resistant layer may include the steps of performing a conventionally known high temperature graphitization process at least once and performing a conventionally known carbon matrix densification process at least once. In the step of performing a high temperature graphitization process, the outer heat resistant layer is heated to a temperature corresponding to a maximum temperature to which the outer heat resistant layer is expected to be exposed when the heat shield is used. The high temperature graphitization process conditions the outer carbon-carbon layer for withstanding severe reentry environments and may be performed more than once, if necessary. In the step of performing a carbon matrix densification process, carbon is deposited in the voids of the carbon-carbon laminate in order to ensure that a fully dense and uniform carbon-carbon matrix is achieved. The outer layer may undergo one or more carbon matrix densification processes until sufficient density of the carbon-carbon outer layer is achieved.
In addition to the steps of disposing one or more layer(s) of phenolic loaded scrim cloth or felt material between the outer and inner layers and simultaneously heating the outer, scrim cloth or felt, and inner layers, the step of attaching may include the step of loading the scrim cloth or felt material with a phenolic resin. This may be accomplished by soaking it in a phenolic resin. The phenolic resin within which the scrim cloth or felt is soaked may include additives such as, for example, graphite fibers or glass frit, suspended therein.
When the insulating inner layer is comprised of a plurality of blocks of the insulating second material, the method of the present invention may include the additional step of bonding adjacent blocks of the second material to one another. This may be accomplished by disposing at least one layer of the phenolic loaded scrim cloth or felt material between adjacent blocks prior to the step of heating the outer, phenolic loaded scrim cloth or felt, and inner layers simultaneously. Alternatively, in lower temperature applications, adjacent blocks of the insulating second material may be bonded to one another using a low temperature adhesive along with a layer of scrim cloth between adjacent blocks.